There is an issue with the gold income when killing monsters that have evolved.
If a monster has evolved and has been left unattended for a while after killing them you won't receive any gold.
I have seen this issue happen mostly with monsters that evolved into boss mobs i.e beetle guard into beetle queen
Artifact of Progression is from the Aetherium mod. I don't use this artifact but a quick look at the code shows that the evolved monsters should drop the same gold as the monster it evolved from.
There is an issue with the gold income when killing monsters that have evolved. If a monster has evolved and has been left unattended for a while after killing them you won't receive any gold. I have seen this issue happen mostly with monsters that evolved into boss mobs i.e beetle guard into beetle queen